1. Field of the Invention
This invention relates generally to electrochemical cells, and especially relates to an integrated membrane, electrode support screen and protector ring that sustains the integrity and structure of the membrane.
2. Brief Description of the Related Art
Electrochemical cells are energy conversion devices, usually classified as either electrolysis cells or fuel cells, including, but not limited to, electrolysis cells having a hydrogen water feed. A proton exchange membrane electrolysis cell functions as a hydrogen generator by electrolytically decomposing water to produce hydrogen and oxygen gases. Referring to FIG. 1, in a typical single anode feed water electrolysis cell 101, process water 102 is reacted at oxygen electrode (anode) 103 to form oxygen gas 104, electrons, and hydrogen ions (protons) 105. The reaction is created by the positive terminal of a power source 106 electrically connected to anode 103 and the negative terminal of a power source 106 connected to hydrogen electrode (cathode) 107. The oxygen gas 104 and a portion of the process water 102' exit cell 101, while protons 105 and water 102" migrate across proton exchange membrane 108 to cathode 107 where hydrogen gas 109, is formed.
The typical electrochemical cell includes a number of individual cells arranged in a stack with fluid, typically water, forced through the cells at high pressures (e.g., a pressure differential of about 30 psi from the cell inlet to the outlet). The cells within the stack are sequentially arranged including a cathode, a proton exchange membrane, and an anode. The cathode/membrane/anode assemblies (hereinafter "membrane and electrode assembly") are supported on either side by packs of screen or expanded metal, which are in turn surrounded by cell frames and separator plates to form reaction chambers and to seal fluids therein. The screen packs establish flow fields within the reaction chambers to facilitate fluid movement and membrane hydration, and to provide mechanical support for the membrane and a means of transporting electrons to and from the electrodes.
As stated above, the screen packs support the membrane and electrode assembly. The membrane is typically only about 0.002-0.012 inches in thickness when hydrated, and the electrodes are thin structures (less than about 0.002 inches) of high surface area noble metals pressed or bonded to either side of the membrane and electrically connected to a power source. When properly supported, the membrane serves as a rugged barrier between the hydrogen and oxygen gases. The screen packs, which are positioned on both sides of the membrane against the electrodes, impart structural integrity to the membrane and electrode assembly. Due to the high pressure differential that exists in an operating cell, however, the membrane and electrode on the low pressure side can be forced into the screen packs.
In the current state of the art, the membrane and electrode are clamped between the cell frames and supported in the active area by a screen pack or other similar porous device. (See FIG. 2) By the nature and design of the assembly, there is a gap 122 between the frame component 124 and the screen pack 126 that the membrane and electrode assembly 128 must span. The pressure on one side of the membrane and electrode assembly 128 is generally higher than the pressure on the other side of the membrane and electrode assembly 128, and the membrane and electrode assembly, must be capable of supporting this pressure differential. The gap 122 between the frame 124 and the screen pack 126 is generally too wide for the membrane and electrode assembly 128 to span and support the pressure differential without perforating. In addition, edges of the screen pack 130 often have burrs and/or other features that are likely to puncture the membrane and/or electrode.
Conventional screen packs have a number of disadvantages and drawbacks. For example, existing screen packs 126 comprise multiple layers of screen material formed from 0.005 inches (0.127 millimeters (mm))-0.010 inches (0.254 mm) thick metal strands having pattern openings of 0.125 inches (3.17 mm) by 0.053 inches (1.35 mm) to 0.071 inches (1.80 mm) (commonly known as 3/0 screen). Under typical operating conditions, a pressure differential of about 390 pounds per square inch (psi) forces the membrane assembly into the openings of the first layer of screen on the low pressure side of the cell. Due to the extrusion of the membrane into this screen layer, the membrane stress in the center of a screen opening increases to about 4,600 psi, while the membrane material has a maximum rating of only about 2,000 psi. Consequently, high axial stresses may force the screen strands into the membrane over time, thereby filling the screen void areas with membrane material. Alternatively, the membrane may rupture, allowing mixing of hydrogen and oxygen gases.
The current state of the art addresses both of the problems associated with pressure differentials and burrs by incorporating a thin metal or polymer protector ring 132 into the electrochemical cell. This ring 132 supports the pressure load imposed on the membrane and electrode assembly 128 over the gap 122 between the cell frame 124 and screen pack 126. The protector ring 132 also provides an impenetrable barrier for the membrane and electrode assembly 128 against the otherwise detrimental screen pack edge features 130.
However, protector rings used in conventional devices pose a number of problems for the construction and operation of the electrochemical cells. First, the protector ring is costly to manufacture, especially when made from materials suitable for use in an electrochemical cell, since much of the material is wasted in the fabrication process. Also, the protector ring is flimsy and difficult to position accurately in the cell assembly, and it is also prone to being dislodged from its intended position during the handling associated with the cell assembly process. Finally, internal cell dynamics associated with repeated pressure cycles can cause relative motion between cell components, which may serve to mislocate the protector ring even after successful cell assembly. This may result in the membrane and electrode ultimately being exposed to the gap and screen pack edge, resulting in perforation and cell failure.
What is needed in the art is a readily manufactured, improved protector ring and screen assembly that provides protection for, and structural integrity to, the membrane and electrode, without adversely affecting the cell's mass flow characteristics.